1. Technical Field
This invention relates generally to a method for increasing engine-out temperature during cold start of a diesel engine to provide fast warm-up of exhaust aftertreatment system components, and more particularly to such a method that includes reducing the mass of the intake air charge and the intake manifold pressure of low-pressure premixed charge compression ignition dominated combustion during cold start.
2. Background Art
The U.S. Environmental Protection Agency (EPA) has set very stringent emissions standards for diesel engines. The standards are directed at reducing smog-causing emissions from all diesel engines, including off-road vehicles, trucks, buses, recreational vehicles, and motor homes. In particular, standards set forth for model year 2007 place significant limits on NOx and particulate matter (PM) emissions that will require changes in both in-cylinder (engine-out) combustion control and exhaust treatment devices.
Several alternative combustion modes, such as low temperature combustion (LTC), premixed charge compression ignition (PCCI), and other variations of standard diesel combustion, are currently being studied for in-cylinder combustion control. Simultaneously, exhaust treatment devices are being developed to synergistically couple with specific combustion modes. Currently, a lean NOx trap (LNT) is the most likely exhaust treatment device for the control of NOx emissions for light-duty diesel vehicles, while diesel particulate filters (DPF) are a near certainty for PM control.
Although these devices currently offer the most promise for controlling exhaust emissions, they are not effective until they are heated to a predefined operating, or activation, temperature. Fast warm-up during cold start is an important issue because if the exhaust treatment devices are not active early in the first phase of the prescribed Federal Test Procedure 75 (FTP-75) driving cycle, the vehicle will exceed the emissions limits.
Several techniques have been proposed for specifically decreasing the warm-up time of exhaust treatment devices. For example, recently published U.S. Pat. No. 6,935,100 B2 granted Aug. 30, 2005 to Manabu Miura for a METHOD FOR WARM-UP CATALYST OF EXHAUST GAS TREATMENT DEVICE describes reducing the excess air ratio of the intake charge during cold start and warm-up. However, the method proposed by Miura requires one or more delay periods after cold start to make certain determinations and adjustments. Moreover, it has been found that controlling excess air, by itself, has limited effect on providing the high in-cylinder (engine-out) temperatures required to quickly heat exhaust treatment devices to their effective operating temperature.
The present invention is directed to overcoming the problems described above with respect to reducing the warm-up time of exhaust treatment devices during cold start and warm-up. It is desirable to have a method for reducing the exhaust flow rate during cold start and warm-up of an engine by immediately reducing the mass of the intake air charge by reducing intake charge pressure during the intake stroke of each combustion cycle and increasing engine fueling. It is also desirable to have a method whereby NOx emissions are reduced during cold start. Moreover, it is desirable to have a method for limiting soot emissions during cold start and warm-up of an engine by using premixed charge compression ignition combustion in which the intake and compression stroke pressures are reduced to enhance fuel vaporization and fuel-air mixing.